Direct Imaging in Real Space and Time with 4D Electron Microscopy

The current methods of detection of nano structures provide insight into the movements but direct real-space and time visualization of modes of oscillations at frequencies pitched in the ultrasonic range (i.e., kilohertz to gigahertz) has not so far been possible.Scientist at Caltech (Ahmed H. Zewail et al), for the first time have reported their observation using four-dimensional (4D) electron microscopy, of the nanomechanical motions of cantilevers.

From the observed oscillations of nanometer displacements as a function of time, for free-standing beams, They were able to measure the frequency of modes of motion and determine Young’s elastic modulus and the force and energy stored during the optomechanical expansions. The motion of the cantilever is triggered by molecular charge redistribution as the material, single-crystal organic semiconductor, switches from the equilibrium to the expanded structure. For these material structures, the expansion is colossal, typically reaching the micrometer scale, the modulus is 2 GPa, the force is 600 μN, and the energy is 200 pJ. These values translate to a large optomechanical efficiency (minimum of 1% and up to 10% or more) and a pressure of nearly 1,500 atm. This has been noted that the observables in the report are real material changes in time, in contrast to those based on changes of optical/contrast intensity or diffraction. The pseudo-one-dimensional molecular material (copper 7,7,8,8-tetracyanoquinodimethane, [Cu(TCNQ)]), which forms single crystals of nanometer and micrometer length scale, has been used as a prototype.Figure show the Atomic scale to macroscale structure of phase I Cu(TCNQ). Shown in the upper panel is the crystal structure as viewed along the a axis (i.e., π stacking axis) and c axis. The unit cell is essentially tetragonal, gray corresponds to carbon, blue corresponds to nitrogen, and yellow corresponds to copper. The lower panel displays a typical selected-area diffraction pattern from Cu(TCNQ) single crystals as viewed down the [011] zone axis along with a micrograph taken in our UEM. The rodlike crystal habit characteristic of phase I Cu(TCNQ) is clearly visible.

In summary, researchers have successfully suggested that with 4D electron microscopy it is possible to visualize in real space and time the functional nanomechanical motions of cantilevers. From tomographic tilt series of images, the crystalline beam stands on the substrate as defined by the polar and azimuthal angles. The resonance oscillations of two beams, micro- and nanocantilevers, were observed in situ giving Young’s elastic modulus, the force, and the potential energy stored. The systems studied are unique 1D molecular structures, which provide anisotropic and colossal expansions. The cantilever motions are fundamentally of two types, longitudinal and transverse, and have resonance Q factors that make them persist for up to a millisecond. The function is robust, at least for 10⁷ continuous pulse cycles (10¹¹ oscillations for the recorded frames), with no damage or plasticity. With these imaging methods in real time, and with other variants, it will be now possible to test the various theoretical models involved in MEMS and NEMS.